Jenson Button's decision to quit BAR and move back to Williams has sent shockwaves through Formula One.

Unusually in a world where secrets are extremely hard to keep, the plot was kept under wraps until it was announced late on Thursday night.

But with BAR out-performing Williams this season and Button apparently under contract with his current team, how and why has he made the switch?

WHY HAS BUTTON DONE IT?

Button said in a statement released by Williams that the team's "massive investment in resources and depth of talent provides the best platform for my future ambitions to be world champion".

But is he right?

Williams have had a disappointing season in 2004, while BAR have been F1's most improved team and, with Renault, were Ferrari's most consistent challengers until the new McLaren was unveiled last month.

The Williams has not been entirely convincing this year

Not only that, but Williams are in a state of technical flux, with the inexperienced Sam Michael taking over from veteran Patrick Head, the team's co-owner, as technical director.

Michael has had a baptism of fire, with disqualifications, massive accidents, internal divisions and the disappointing performance of a heavily revised version of Williams' FW26 car marking the start of his reign.

It would be unfair to judge the potential of one of F1's blue-riband teams on a few races, and Michael's influence will be seen more clearly when Williams produce their first car under his leadership next year.

Nevertheless, many would contend that Button's short-term future might be better served at BAR - and at 24, he has plenty of time to re-join Williams later in his career.

HOW HAS HE DONE IT?

BAR boss Richards says he is determined to hang on to Button

BAR had taken up their option on Button's services for 2005, but the driver's management team have used as an excuse to exercise their wish to move to Williams the claim that the contract is not valid because of a technicality.

That technicality is believed to be based on the small print of BAR's recently-extended contract with engine supplier Honda.

The contract is understood to reflect Honda's concerns about BAR's future with the possibility of a global ban on tobacco advertising being brought in over the next few years - the team are owned, and almost entirely funded by, tobacco giant BAT, which has been looking for buyers.

Button's contract contains a clause that allows him to leave the team if BAR risk losing their Honda engines.

WILL THE MOVE STICK?

Probably - if Button wants to move to Williams, ultimately there is little BAR can do to stop him. Under European legislation that would be counted as a restraint of trade.

However, BAR boss David Richards says the team will contest the move, and the row is likely to go before F1's contracts recognition board.

If this rules in BAR's favour, then Williams will have to pay their rivals compensation for taking Button away.

Either way, Button's last race for his current team is likely to be this year's season-ending Brazilian Grand Prix.

IS THERE A PRECEDENT FOR THIS?

Yes - a number of top drivers have left a team to which they are still apparently under contract to move elsewhere. Alain Prost went from McLaren to Renault in 1980; Ayrton Senna left Toleman for Lotus in 1984; and Michael Schumacher moved from Jordan to Benetton after just one race in 1991.

Schumacher's move led to the establishment of the contracts recognition board, which subsequently ruled on a dispute between Williams and McLaren over David Coulthard at the end of 1994.

In that case, the CRB decided in Williams' favour, McLaren accepted the move and Coulthard raced for Williams throughout 1995 before moving to McLaren in 1996.

In what some would see as a parallel with Button, Coulthard's decision to sign for McLaren was based partly on the team's long-term potential rather than short-term competitiveness. And many believe he would have been a two-time world champion had he stayed with Williams in 1996 and 1997.

WHO WILL REPLACE BUTTON AT BAR?

With Button set to leave the team, BAR need to find a top-line replacement for him in 2005 and the only obviously available candidate who fits that description is David Coulthard.

Coulthard could find an opening at BAR after Button's departure

The Scot has lost his McLaren seat to Juan Pablo Montoya and until now only had less-than-appealing options at Jaguar and Sauber for 2005.

The 33-year-old Scot has not been entirely convincing in the last two seasons, mainly because he has struggled to come to terms with the one-lap qualifying system.

But he is a proven winner and by far the best option for the likely vacancy unless Jarno Trulli has not yet signed for Toyota, as most believe he has, or Mika Hakkinen is serious in his apparent desire to make a comeback.

Other options include BAR's test driver Anthony Davidson, who has impressed in Friday practice this season, Olivier Panis, who is to be dropped by Toyota, Jordan's Nick Heidfeld and Williams test driver Antonio Pizzonia.

Brazilian Cristiano da Matta is also free after being dropped by Toyota on Thursday, but is unlikely to appeal.